In general, Dawnguard's requirements, in terms of level and skills, are lower than Dragonborn's. I'd say that level 20 is good for Dawnguard while level 30 is good for Dragonborn. Actually, you can probably get away with 15 for Dawnguard and 25 for Dragonborn, depending on how you've built your character.

So if you're not quite there for Dragonborn yet, go for Dawnguard and level up through the DLC. Then go off and do Dragonborn.